1. Field of the Invention
The present invention relates to an improved latch assembly which operates with a phased array radar antenna to fixedly retain the radar antenna in any one of several predetermined positions.
2. Description of the Prior Art
The phased array radar antenna will usually be located in the nose cone of an aircraft. Its special requirements make it unique among conventional radar antennas. Unlike conventional radar which physically, continuously sweeps the horizon in a 180.degree. arc, the phased array radar antenna is inset with phase control modules. These modules electronically scan the horizon while the antenna remains fixed. The phased array radar antennas primary positioning requirement is that it must remain in a locked, stable, fixed position for the duration of its electronic scan.
A phased array radar antenna utilizes a latch pin to hold it in any one of several desired positions, typically three. The latch assembly is designed to provide a spring load on the pin which compensates for tolerances of the detent ring and assembly of the latch onto the antenna. The end of the pin preferably utilizes a Morse taper which provides the most shallow taper which will not then be jammed in place due to friction forces. The taper also assures that the fit between the pin and rotating parts will be tight and not allow relative motion.
A problem arises when the fixed phased array radar antenna is disturbed, thereby loading the end of the pin and creating minimal clearances between the detent pin and the latch pin housing. Thus a minimal clearance allows the detent latch pin to move which in turn allows the antenna to move. By design, approximately 1/4 milliradian of antenna motion is allowed without seriously affecting antenna performance. Part wear, lack of tolerance between the parts, however, cannot allow this built-in clearance to become larger. In practice, one antenna had been measured with up to 1 milliradian of free motion.
A further problem, and perhaps a more serious one, is the effect of the latch pin clearance on the dynamics of the antenna. In phase array radar antenna, for example, the radio frequency performance places an important requirement that the antenna be as rigid as possible so that the resonant frequency of the antenna structure can be as high as possible. Any free motion in the structural system results in degradation of the antenna dynamics and, in turn, of the antenna RF performance.
The problem to be solved, therefore, is the problem of reducing the clearance between the pin housing essentially to zero without degrading the performance of the latch itself. It would be desirable to minimize any modifications to the latch assembly once in place and the antenna is in production. The preferred embodiment of the disclosed invention, the improved latch assembly, accomplishes both desired effects.
To resolve this problem of zero housing and latch pin clearance, several approaches may be taken. One approach is to machine the pin and housing to extremely tight tolerances. This would increase the cost and technical risks, however, without ever achieving zero clearance. Another approach would be to place a heater upon the pin, causing the pin to expand due to thermal expansion. The problem with the thermal expansion technique, however, is that a sufficient time must pass after heating for the pin to cool before it can be successfully retracted.